1. Field of the Invention
The present invention relates to a control method and a control system for an automatic transmission.
2. Related Art
The conventional automatic transmissions of the prior art include a torque converter acting as a fluid coupling for receiving the rotation generated by an engine, and a gear change unit for changing the speed of the rotation transmitted from the torque converter. The gear change unit has at least one planetary gear set composed of a plurality of gear elements and effects a shift in accordance with a shift pattern which is preset to be followed in accordance with vehicle speed, the throttle opening and so on.
In using the prior art automatic transmission the driver can select a (parking) P-range, a (reverse) R-range, a (neutral) N-range, a (drive) D-range, a (second) S-range, a (low) L-range and so on. If the range is switched from the N-range to the D-range by the shift lever, for example, the rotation of the engine at idle is transmitted through the torque converter to the transmission, to produce a creep phenomenon in which the vehicle will advance little by little without any depression of the accelerator pedal.
When, therefore, a range such as the D-range, the S-range or the L-range (hereinafter each referred to as a "forward range") for moving the vehicle forward is selected, while the vehicle is substantially at a standstill, the forward clutch to be applied in forward running, i.e., a first clutch, is released or brought into slipping engagement to establish a pseudo-neutral state (hereinafter called the "neutral control state") so that the aforementioned creep phenomenon may be prevented.
When the above-mentioned neutral control state is to be established, hill-holding control is simultaneously applied to prevent reverse movement of the vehicle, against the will of the driver, on an incline (as disclosed in U.S. Pat. No. 4,648,289). In the hill-holding mode, one of a plurality of frictional engagement elements of the transmission is engaged to prevent the output shaft of the transmission from rotating in reverse. However, if the accelerator pedal is depressed to start the vehicle while in the hill-holding mode, the application of the first clutch and the release of the hill-holding control are simultaneously effected. Since, however, there is an abrupt shift from the state in which the reverse motion of the vehicle is blocked by the hill-holding control, to a state in which the hill-holding control is inactive, the vehicle may move backward, against the will of the driver, when an attempt is made to start the vehicle uphill on a steep gradient.
Specifically, when the neutral control and the hill-holding control are not in effect facing uphill on a steep incline, the driver must exert a high depressing force on the brake pedal to stop the vehicle. However, when the vehicle is to be started, the driver must gradually reduce the depressing force on the brake pedal. The driver then becomes aware of the necessity of quickly applying a high force of depression to the accelerator pedal, before the brake pedal is fully released, because the vehicle would otherwise gradually roll backward. As a result, if depression of the accelerator pedal is not too late or too little, it is possible to start without significant backward movement of the vehicle.
On the other hand, in the aforementioned control system of the prior art, the blocking force which holds against backward movement of the vehicle is provided by the hill-holding control until the brake pedal is fully released. However, when the brake pedal is fully released, the blocking force is suddenly removed by cancellation of the hill-holding control. As a result, if the driver is slow to become aware of necessity of above-mentioned operation of the accelerator pedal, the vehicle will roll backward, contrary to the driver's intention.